1. Field of the Invention
The present invention relates to a reflective guest-host liquid crystal display (LCD) device having a low cost, simple structure which realizes both high contrast ratio and high reflectivity.
2. Description of the Related Art
An LCD device including a backlight has been widely used as a display device of a desk top personal computer, a portable type personal computer and PDA (Personal Digital Assistants). The latter two portable devices require an LCD device which is thin, light weight and has low power consumption. Therefore, when the LCD device requiring the backlight is used in the portable devices, a power load of the portable equipment is increased.
In place of the LCD device requiring the backlight, a reflective LCD device which uses a guest-host layer and does not require the backlight has been developed as the LCD device of the personal devices. The xe2x80x9cguestxe2x80x9d means dichroic dye added to the liquid crystal and the xe2x80x9chostxe2x80x9d means the liquid crystal. The dichroic dye indicates a high absorbance along its longitudinal direction or a long axis. In the guest-host scheme, an alignment direction of the liquid crystal molecules is controlled by an electric field generated by an applied voltage, and the dichroic dyes are simultaneously aligned in the same direction as that of the liquid crystal molecules. If the liquid crystal has a positive anisotropy of dielectric constant, when the electric field is turned off, the guest-host layer sandwiched between two glass substrates is aligned into the homogeneous structure. When the homogeneous structure is illuminated by a visible light, the dichroic dye absorbs a linear polarization component in a plane parallel to its longitudinal direction and passes a linear polarization component in a plane perpendicular to the longitudinal direction. When the electric field is applied, the liquid crystal molecules and the dichroic dyes are aligned in a direction perpendicular to the surface of the glass substrates, and all the incident light passes through the guest-host layer to provide a bright state.
Both high reflectivity at the bright state and high contrast ratio are key factors to the improvement of the reflective guest-host LCD display. Hereinbelow, four types of conventional reflective guest-host LCD devices using the liquid crystal of the positive anisotropy of dielectric constant are described.
In the first-type reflective guest-host LCD device, a single guest-host layer of the homogeneous structure is used. When the voltage applied to the guest-host layer is turned off to provide a dark state, the linear polarization component of the incident light in a plane perpendicular to the longitudinal direction of the dichroic dye passes through the guest-host layer and is reflected toward the guest-host layer by a reflector disposed at the bottom side of the guest-host layer. This reflected linear polarization component passes through the guest-host layer and is observed by a user. Hence, in the dark state, although one linear polarization component of the incident light is suppressed, the other linear polarization component is not suppressed, so that high contrast ratio is not obtained.
In the second-type reflective guest-host LCD device called a xe2x80x9cWhite-Taylor mode devicexe2x80x9d, the liquid crystal molecules and the dichroic dyes are twisted more than 200 degrees between the two glass substrates when the voltage is turned-off, to absorb all the linear polarization components to improve the contrast ratio. However, this type of LCD device does not realize the high reflectivity at the bright state or at the turn on of the voltage. Hence, both the first and second LCD device types cannot simultaneously realize both high contrast ratio and high reflectivity.
In the third-type reflective guest-host LCD device called a xe2x80x9cdouble layer guest-host LCD devicexe2x80x9d, a space between the two glass substrates is separated into an upper compartment and a lower compartment by a transparent separator placed between the two glass substrates. At the turn off of the voltage or the electric field, the guest-host layer of the homogeneous structure in the upper compartment is aligned in one direction, such as a direction parallel to the plane of this page, and the guest-host layer of the homogeneous structure in the lower compartment is aligned in a direction perpendicular to the above one direction, such as a direction passing through the plane of this page. In the dark state, when the electric field is turned off, although the linear polarization component of the incident light in the plane perpendicular to the longitudinal direction of the dichroic dye passes through the guest-host layer in the upper compartment, this passed linear polarization component is suppressed by the guest-host layer in the lower compartment since the plane of the passed linear polarization component matches the plane parallel to the longitudinal axis of the dichroic dyes in the guest-host layer in the lower compartment. Although the third-type LCD device can perfectly block the light at the dark state, a parallax is generated, the structure is complicated, and the fabrication cost is expensive in comparison with the single guest-host layer.
In the fourth-type reflective guest-host LCD device, a reflector, a quarter-wave plate (xcex/4 plate) and an ITO electrode are formed on an inside surface of a lower glass substrate in this order. The ITO is formed on an inside surface of a upper glass substrate. The guest-host layer is sandwiched between the two glass substrates. The linear polarization component of the incident light in the plane perpendicular to the longitudinal direction of the dichroic dye passes through the guest-host layer. During the reflection by the combination of the xcex/4 plate and the reflector, the plane of the passed linear polarization component is rotated by 90 degrees and this 90-degrees-rotated linear polarization component is blocked by the guest-host layer, so that the light is perfectly blocked at the dark state and both high contrast ratio and high reflectivity are realized. However, in this fourth-type device, it is difficult and expensive to design the xcex/4 plate which passes the visible light of a range 450 nm to about 700 nm, and a complicated assembly process is required.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to provide a reflective guest-host LCD device having a low cost, simple structure which realizes both high contrast ratio and high reflectivity.
A reflective liquid crystal display device in accordance with a first aspect of the present invention includes a first transparent substrate, a first electrode disposed on one surface of the first transparent substrate, a second transparent substrate having a first surface and a second surface, and a second electrode disposed on the first surface of the second transparent substrate, wherein the first transparent substrate and the second transparent substrate are so arranged that the first electrode and the second electrode face each other and a space is provided between the first electrode and the second electrode, a guest-host liquid crystal layer is disposed in the space, the first electrode and the second electrode define a plurality of picture element positions, and a reflector unit having a shape of a trigonal pyramid is disposed at each of the plurality of picture element (pixel) positions on the second surface of the second transparent substrate.
The trigonal pyramid preferably includes a bottom plane of a shape of a right-angled isosceles triangle having two shorter edges and one longer edge, a first reflecting plane extending from one of the shorter edges, a second reflecting plane extending from the other of the shorter edges, and a third reflecting plane extending from the longer edge, an angle between the bottom plane and the first reflecting plane is approximately 45 degrees, an angle between the bottom plane and the second reflecting plane is approximately 45 degrees, and an angle between the bottom plane and the third reflecting plane is approximately 87 degrees to about 90 degrees.
Preferably, the second transparent substrate and the reflector units are integrally formed by a transparent material having a refractive index equal to or larger than approximately 1.3.
Preferably, one of the shorter edges of the right-angled isosceles triangle is aligned to a rubbing direction for the guest-host liquid crystal layer. Preferably, a light scattering layer is disposed between the one surface of the first transparent substrate and the first electrode.
Preferably, a color filter layer is disposed between the light scattering layer and the first electrode.
A reflective liquid crystal display device in accordance with a second aspect of the present invention includes a first transparent substrate, a first electrode disposed on one surface of the first transparent substrate, a second transparent substrate having a first surface and a second surface, and a second electrode disposed on the first surface of the second transparent substrate, wherein the first transparent substrate and the second transparent substrate are arranged so that the first electrode and the second electrode face each other and a space is provided between the first electrode and the second electrode, a guest-host liquid crystal layer is disposed in the space, the first electrode and the second electrode define a plurality of picture element positions, and two reflector units each of which has a shape of a trigonal pyramid are disposed at each of the plurality of picture element positions on the second surface of the second transparent substrate.
The trigonal pyramid preferably includes a bottom plane of a shape of a right-angled isosceles triangle having two shorter edges and one longer edge, a first reflecting plane extending from one of the shorter edges, a second reflecting plane extending from the other of the shorter edges, and a third reflecting plane extending from the longer edge, an angle between the bottom plane and the first reflecting plane is approximately 45 degrees, an angle between the bottom plane and the second reflecting plane is preferably about 45 degrees, an angle between the bottom plane and the third reflecting plane is preferably about 87 degrees to about 90 degrees, and the two reflector units at each of the plurality of picture element positions are so disposed that the third reflecting plane of one of the two reflector units is adjacent to the third reflecting plane of the other of the two reflector units.
Preferably, a top point of the one reflector unit and a top point of the other reflector unit are separated by an air gap of about 2 xcexcm to about 10 xcexcm. A top point of the one reflector unit and a top point of the other reflector unit are more preferably separated by an air gap of about 5 xcexcm to about 8 xcexcm.
A reflector unit array for a reflective guest-host liquid crystal display device in accordance with a third aspect of the present invention includes two reflector units disposed at each of a plurality of picture element positions of the liquid crystal display device for reflecting an incident light into the liquid crystal display device, wherein each of the reflector units has a shape of a trigonal pyramid.
The trigonal pyramid includes a bottom plane of a shape of a right-angled isosceles triangle having two shorter edges and one longer edge, a first reflecting plane extending from one of the shorter edges, a second reflecting plane extending from the other of the shorter edges, and a third reflecting plane extending from the longer edge, an angle between the bottom plane and the first reflecting plane is preferably approximately 45 degrees, an angle between the bottom plane and the second reflecting plane is preferably approximately 45 degrees, an angle between the bottom plane and the third reflecting plane is preferably approximately 87 degrees to about 90 degrees, and the two reflector units at each of the plurality of picture element positions are so disposed that the third reflecting plane of one of the two reflector units is adjacent to the third reflecting plane of the other of the two reflector units.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 11-359670, filed Dec. 17, 1999, which is expressly incorporated herein by reference in its entirety.